Don't try too hard so that your participation doesn't come across as forced/fake. You need to be genuinely interested in the subject you're posting about. Preferably knowledgeable as well, at least to the point where you can ask a reasonable question. If there is anything that you're good at IRL - you can start with that, or something that's close to it. There are boards and threads on this forum for just about anything.

However you should avoid generic mega-threads like Token goes down. Wait or sell? - you have no chance to earn merits on such a "yes/no" thread, particularly on page 113. Anything that could be said on that topic has already been said and I bet your annual bounty earnings that you didn't read the thread before posting in it.